1. Field of the Invention
The present invention relates to an display apparatus and a method of driving the display apparatus and more particularly to a technique which is effective for application to an display apparatus in which a plurality of luminance modulation elements are arranged in a matrix.
2. Description of Relates Art
The display apparatuses in which a plurality of luminance modulation elements are arranged in a matrix include liquid crystal displays, field emission displays (FED), organic electroluminescence displays and the like. The luminance modulation element is adapted to change luminance depending on the applied voltage. In this specification, the luminance means transmittance or reflectance in the case of the liquid crystal display, and brightness of emission light in the case of displays using light emitting elements, such as the field emission display or the organic electroluminescence.
The displays described above have a merit capable of reducing the thickness of the display apparatus.
Accordingly, they are effective particularly as portable display apparatuses.
Those showing the background described above can include, for example, Patent Document 1, Non-Patent Document 1, Non-Patent Document 2, Non-Patent Document 3, Non-Patent Document 4, and Non-Patent Document 5. The documents will be described specifically later.
[Patent Document 1] JP-A No. 162927/2002
[Non-Patent Document 1] 1997 SID International Symposium Digest of Technical Papers, pp. 1073-1076 (issued, May 1997)
[Non-Patent Document 2] 1999 SID International Symposium Digest of Technical Papers, pp. 372-375 (issued, May 1999)
[Non-Patent Document 3] EURODISPLAY'90, 10th International Display Research Conference Proceedings (vde-verleg, Berlin, 1990), pp. 374-377
[Non-Patent Document 4] Japanese Journal of Applied Physics, vol. 34, part 2, No. 6A, pp. L705-L707 (1995)
[Non-Patent Document 5] Japanese Journal of Applied Physics, vol. 36, part 2, No. 7B, pp. L939-L941 (1997)
In a portable display apparatus, it is an important characteristic that the power consumption is small. Further, also in an installed type or a desk top type display apparatus, it is desirable that the power consumption is small with a view point of effective utilization of energy, or with a viewpoint of lowering the heat generation in the display apparatus.
However, in the prior art, large power in charge and discharge to and from the electric capacitance of the luminance modulation element caused increase in the power consumption.
In order to solve the problem, a method of decreasing the charge/discharge power by setting the non-selected electrode to high impedance in an display apparatus in which unipolar luminance modulation elements are arranged in a matrix has been disclosed, for example, in Patent Document 1 by the present applicant.
According to this method, the non-selected scanning line is set to a higher impedance state than the selected scanning line to decrease the load capacitance of the data line circuit substantially smaller thereby decreasing the charge/discharge power. On the other hand, in this method, since the potential on the electrode at the high impedance state is in a floating state, the potential is not constant. That is, an accidental voltage (induced voltage) is induced to the electrode at the high impedance state.
The example of disclosure described above discloses an image display method in which the induced voltage less tends to give an effect on the displayed image by combination of luminance modulation characteristics of unipolar luminance modulation elements, based on that the induced voltage tends to have a specified polarity.
However, since the potential of the electrode in the high impedance state is indefinite in view of principle, an accidental voltage is sometimes induced thereby possibly giving an effect on the display state.
In view of the problem, it has been disclosed a method of controlling the polarity of the induced voltage by setting only the scanning line adjacent with the selected scanning line to a low impedance state thereby controlling the polarity of the induced voltage in Patent Document 1 by the present applicant.
However, since the electrode in the high impedance state is indefinite in view of the principle, an accidental voltage is sometimes induced even in a case of using the method disclosed in the known example described above to possibly give an undesired effect on the display state.
For describing the feature of the invention, description is to be made specifically for the subject of the driving method disclosed so far. Description is to be made to an example of using a thin-film electron emitter and a phosphor in combination as a luminance modulation element.
FIG. 2 is a view showing a schematic constitution of a matrix for luminance modulation elements.
A luminance modulation element 301 is formed at each intersection between row electrodes 310 and column electrodes 311.
While FIG. 2 shows an example of 3 rows×3 columns, the luminance modulation elements 301 are arranged actually by the number of pixels constituting a display apparatus or by the number of sub-pixels in the case of a color display apparatus.
That is, in a typical example, the number N of rows and the number M of columns are typically: N=hundreds to thousands of rows and M=hundreds to thousands of columns, respectively.
In the case of color image display, a combination of each of sub-pixels of red, blue and green forms one pixel. In the present specification, those corresponding to sub-pixels in a case of color image display may also sometimes be referred to as “pixels”. Alternatively, pixels in the case of monochrome display and sub-pixels in the case of color display are sometimes collectively referred to as “dot”.
FIG. 3 is a timing chart for explaining an conventional driving method of an display apparatus. A negative pulse at an amplitude (Vk) (scanning pulse 750) is applied to one of row electrodes 310 (selected row electrode) from a row electrode driving circuit 41 and, at the same time, a positive pulse at an amplitude Vdata (data pulse 760) is applied to some of column electrodes 311 (selected column electrodes) from a column electrode driving circuit 42.
Since a voltage sufficient to emit light is applied to the luminance modulation element 301 on which two pulses are superimposed, the element emits light.
Since no sufficient voltage is applied to the luminance modulation element 301 not applied with the positive pulse with an amplitude (Vdata), it does not emit light.
The row electrode 310 to be selected, that is, the row electrode 310 applied with the scanning pulse is selected successively and the data pulse applied to the column electrode 311 is also changed corresponding to the line.
When all the lines are thus scanned in a 1-field period, images corresponding to arbitrary images can be displayed.
In the matrix type display apparatus, a dissipation power consumption in the driving circuit causes a problem. The dissipation power consumption is a power consumed for charging and discharging electric charges to and from a capacitance of a driven element. The dissipation power does not contribute to light emission.
Capacitance per one luminance modulation element 301 is assumed as Ce. As can be seen from FIG. 2, a load capacitance of NCe is connected to each column electrode driving circuit 42. Accordingly, in a case of applying data pulses to the luminance modulation elements by the number of m per one line, a load capacitance of mNCe is connected in the column electrode driving circuit 42 in total. The electric power for charging and discharging to and from the load capacitance is the dissipation power consumption described above.
Assuming the number of refreshing screen for one sec (field frequency) as f, the dissipation power in the column electrode driving circuit 42 (Pdata) is represented by the following equation (1):Pdata=f·N2·m·Ce·(Vdata)2  (1)
Then, it is considered for a case where scanning lines other than those scanning lines to be applied with scanning pulses (the latter is referred to as scanning lines in the selected state) are set to a floating state (FIG. 4). In this state, since the load capacitance of the data line circuit is substantially decreased, the dissipation power in the column electrode driving circuit 42 is decreased. The scanning line in the non-selected state can be set to the floating state by setting the scanning line in the non-selected state to a high impedance state. The method of decreasing the dissipation power by the method described above is disclosed, for example, in the Patent Document 1 by the present applicant.
The load capacitance in the entire data line circuit in this case is represented by the following equation (2):
                                          C            col                    ⁡                      (            m            )                          =                              {                          m              +                                                                    m                    ⁡                                          (                                              M                        -                        m                                            )                                                        ⁢                                      (                                          N                      -                      1                                        )                                                  M                                      }                    ⁢                      C            e                                              (        2        )            
It takes a maximum value at m=M/2. In the driving method of connecting the scanning line in the non-selected state to a low impedance, the load capacitance of the data line takes a maximum value at m=M and, compared with this maximum value, the maximum value in the driving method of setting the scanning line in the non-selected state to the high impedance state is decreased to ¼. On the other hand, since setting the non-selected scanning lines to the floating state makes the potential of the scanning lines unstable, it may possibly gives an effect on displayed images. However, as disclosed in the Patent Document 1 by the present applicant, the polarity of the voltage induced to the non-selected scanning line induces a potential in a specified direction. That is, the voltage VF,scan induced to the non-selected scanning line is represented by the following equation (3).VF,scan=(m/M)Vdata=xVdata  (3)where x=m/M is a ratio for the number of luminance modulation elements in the ON state in one line and it is called as a lighting ratio. Vdata represents an amplitude voltage for the data pulse. The lighting ratio x is positive or zero. Accordingly, when Vdata is a positive voltage as shown in the driving waveform in FIG. 4, the induced voltage VF,scan is positive or zero. In FIG. 4, since the luminance is modulated when a negative voltage is applied to the scanning line, the induced voltage has a polarity which does not cause the luminance modulation. Accordingly, it is possible to decrease the effect of the induced voltage on the display images sufficiently by using unipolar luminance modulation elements and connecting them in the direction of not modulating the luminance by the polarity of the induced voltage.
The “unipolar” luminance modulation element is to be described.
An element that does not emit light when applied with a voltage of reverse polarity, that is, an element not taking the selected state for the luminance modulation state is referred to as “unipolar luminance modulation element” in a more general expression, in the sense that the luminance is modulated only by applying a voltage of the positive polarity. On the contrary, an element that emits light or takes the selected state for the luminance modulation state also when the voltage at reverse polarity is applied is referred to as “bipolar luminance modulation element” in the sense that the luminance is modulated by applying a voltage of either of two polarities: positive and negative polarities.
As apparent from the foregoing description, “not modulating luminance at reverse polarity” may be at such an extent as not causing crosstalk of displayed images even when a voltage at the reverse polarity is applied. Even for an element that modulates the luminance slightly upon application of a voltage at reverse polarity, if the state of luminance modulation is within a range not visible to human eyes or not causing a problem as the display apparatus, this can be regarded substantially as “not modulating luminance”. The element can therefore be regarded as “unipolar” luminance modulation element.
The unipolar luminance modulation element is to be described further in details. Luminance modulation elements having luminance-voltage characteristics shown in FIG. 5A and FIG. 5B are to be considered. Description is to be made to an example of a light emission element as the luminance modulation element. In FIGS. 5A and 5B, the ordinate indicates the luminance, that is, brightness in the case of the light emitting element, while the abscissa indicates a voltage applied to the luminance modulation element. In the characteristics shown in FIG. 5A, when a voltage at positive polarity is applied, the luminance increases, whereas when a voltage at negative polarity is applied, the luminance is substantially zero. That is, the luminance modulation element having the characteristics shown in FIG. 5A is unipolar. On the other hand, in FIG. 5B, the luminance changes also in a case of applying a voltage at negative polarity. That is, the luminance modulation element having the characteristics shown in FIG. 5B is bipolar.
Considered is a case of constituting a matrix: N rows×M columns with luminance modulation elements and applying the driving voltage shown in FIG. 4. A scanning pulse at a negative voltage Vk is applied to the selected line to render it into a “half-selected” state. A data pulse at a positive voltage Vdata is applied to the data lines for the luminance modulation elements which are intended to be lighted among the selected line. Accordingly, a voltage: Vdata−Vk=|Vdata|+|Vk| is applied to the luminance modulation elements at the intersections between the selected scanning line and the selected data lines, by which the luminance modulation elements emit light (point C in the figure).
In this case, a voltage: VF,scan represented by the equation (3) is induced to the scanning line in the non-selected state. Accordingly, a voltage: −VF,scan is applied to the luminance modulation elements at the intersections between the non-selected scanning line and the non-selected data lines (point D in the figure). In a case of the bipolar luminance modulation element of FIG. 5B, it slightly emits light by the induced voltage: VF,scan (point D in the figure). That is, not-intended luminance modulation element emits light. Accordingly, this disturbs displayed images. This is a problem in a case where the non-selected scanning line is set to high impedance.
The problem can be overcome by using the unipolar luminance modulation element. In a case of the unipolar luminance modulation element shown in FIG. 5A, it does not emit light even when −VF,scan is applied (point D in the figure). Accordingly, displayed image is not disturbed even when the non-selected scanning line is set to high impedance.
In the foregoings, description has been made to a case that the scanning pulse is a negative voltage and the data pulse is a positive voltage. It will be apparent that the situation is quite identical in a case where the scanning pulse is a positive voltage and the data pulse is a negative voltage. The equation (3) is valid also in this case, in which the voltage VF,scan induced to the scanning electrode is a negative voltage. Since this is at a polarity reverse to the luminance modulation element, no erroneous displayed image occurs by using the unipolar luminance modulation element as described above.
Examples of the bipolar luminance modulation element can include liquid crystal elements and thin film inorganic electroluminescence elements. The unipolar luminance modulation element can include, for example, an organic electroluminescence elements or electron emitting elements in combination with phosphors.
The organic electroluminescence element is also referred to as an organic light emitting diode, which has a diode characteristic of emitting light upon application of a forward voltage but not emitting light upon application of a voltage at reverse polarity. The organic electroluminescence element is described, for example, in Non-Patent Document 1. The polymer type organic electroluminescence element is described in Non-Patent Document 2.
An example of the luminance modulation element comprising a phosphor and an electron emitting element in combination is described, for example, in Non-Patent Document 3. In this example, the electron emitting element comprises an electron emitting emitter-tip and a gate electrode for applying an electric field to the emitter-tip. When a voltage positive to the emitter-tip is applied to the gate electrode, electrons can be emitted from the emitter-tip to emit light from the phosphor but the electrons are not emitted in a case of applying a negative voltage. That is, this is a unipolar luminance modulation element.
As described above, Patent Document 1 by the present applicant discloses that the effect of the induced voltage on the displayed images can be decreased by using the unipolar luminance modulation element.
However, a voltage of forward polarity of the luminance modulation element is sometimes induced to the scanning electrode in the floating state.
For example, when a scanning pulse is applied, a voltage of forward polarity is sometimes induced to the adjacent scanning electrode due to capacitive coupling between the adjacent scanning electrodes. The Patent Document 1 by the present applicant discloses a method of rendering only the scanning line adjacent with the scanning line to be applied with the scanning pulse to the low impedance state in order to prevent this.
However, in the method disclosed in the Patent Document 1, generation of the induced voltage of the forward polarity is not always inhibited. The present invention provides a method of minimizing the occurrence of the induced voltage of the forward polarity even in such a case, thereby minimizing the effect on the displayed images in a display apparatus constituted with unipolar luminance modulation elements.